This invention relates to a novel process for the preparation of xcex1(2,4-disulfophenyl)-N-tert-butylnitrone and pharmaceutically acceptable salts thereof. These compounds have previously been disclosed as being useful as medicaments. Such compounds are alternatively named as 4-[(tert-butylimino)methyl]benzene-1,3-disulfonic acid N-oxide derivatives.
U.S. Pat. No. 5,488,145 discloses xcex1-(2,4-disulfophenyl)-N-tert-butylnitrone, pharmaceutically acceptable salts thereof and related pharmaceutical compositions. U.S. Pat. No. 5,475,032 discloses the use of such compositions in the treatment of stroke and of progressive central nervous system function loss conditions. U.S. Pat. No. 5,508,305 discloses the use of such compositions for ameliorating the side effects caused by oxidative damage resulting from antineoplastic disease treatment. Similar disclosures are also made in WO 95/17876. U.S. Pat. No. 5,780,510 discloses the use of these same compounds in the treatment of concussion.
Various methods are available for the synthesis of nitrones. The most often used method involves the usually uncatalysed condensation reaction of a hydroxylamine derivative with an aldehyde or ketone (J. S. Roberts in D. H. R. Barton and W. D. Ollis, Comprehensive Organic Chemistry, Volume 2, pp. 500-504, Pergamon Press, 1979; R. D. Hinton and E. G. Janzen, J. Org. Chem., 1992, 57, pp. 2646-2651). The utility of this reaction is impaired by its susceptibility to steric hindrance, slow reaction rates, and, in certain cases, by the relative inaccessibility and/or instability of the hydroxylamine starting material. The latter problems can sometimes be overcome by in situ generation of the required hydroxylamine by reduction of a more readily available compound such as the corresponding nitro derivative. This general methodology is employed in the above-described patents where the preparation of xcex1(2,4-disulfophenyl)-N-tert-butylnitrone is described as involving the reaction of 4-formyl-1,3-benzenesulfonic acid with N-tert-butylhydroxylamine in refluxing methanol for approximately 18 hours.
xcex1-(2-Sulfophenyl)-N-tert-butylnitrone has been prepared by reaction of 2-formylbenzenesulfonic acid sodium salt with N-tert-butylhydroxylamine in refluxing ethanol for 2 days (E. G. Janzen and R. V. Shetty, Tetrahedron Letters, 1979, pages 3229 to 3232).
A modification of this type of methodology for the manufacture xcex1-phenyl-N-methylnitrone has been described in French Patent 1,437,188 to E.I. DuPont de Nemours and Co.
We now disclose a novel process that possesses significant advantages for the preparation of xcex1-(2,4-disulfophenyl)-N-tert-butylnitrone and salts thereof and is also particularly suited to large-scale production.
In one aspect, this invention provides a process for the preparation of a compound of general formula (I) 
wherein each R independently represents SO3H or a salt thereof.
This process involves reaction of an aldehyde of general formula (II) 
wherein R is as defined above,
with N-tert-butylhydroxylammonium acetate (III)
(CH3)3CNHOH CH3CO2Hxe2x80x83xe2x80x83(III)
In a second aspect, this invention provides a method for preparing and recovering a compound of general formula (I). In the first step of this process, the compound is prepared as just described. In a subsequent step, the compound is isolated.
In this process, an aldehyde of general formula (II) is reacted with N-tert-butylhydroxylammonium acetate to form an xcex1-(2,4-disulfophenyl)-N-tert-butylnitrone compound of general formula (I). The compounds of general formulae (I) and (II) may be acids or they may be salts.
Salts of compounds of formula (1) above may be formed by reacting the free acid (wherein R represents SO3H), or another salt thereof, with two or more equivalents of an appropriate base, using methods that are well known in the art.
The salts of compounds of formulae (I) and (II) referred to above will normally be those formed with pharmaceutically acceptable cations. The cation may be a monovalent material such as sodium, potassium, lithium, ammonium, alkylammonium or diethanolammonium. Alternatively, it may be a polyvalent cation such as calcium, magnesium, aluminium or zinc. It may also be a mixed salt formed with a polyvalent cation such as calcium or magnesium in combination with a pharmaceutically acceptable anion such as halide (for example chloride), phosphate, sulphate, acetate, citrate or tartrate.
The two R""s in these formulae are usually the same. However, they can be independently selected from the possibilities just enumerated.
It is preferred that the two R""s in formulae (I) and (II) above be the same and each represents SO3xe2x88x92Na+.
N-tert-Butylhydroxylammonium acetate is disclosed in co-pending PCT patent application WO 00/02848.
The aldehydes of general formula (II) are either commercially available or may be prepared from commercially available materials using methods that are well known in the art. Commercial 4-formyl-1,3-benzenedisulfonic acid disodium salt (II; Rxe2x95x90SO3xe2x88x92Na+) typically contains small but significant amounts of the corresponding benzyl alcohol and the corresponding benzoic acid derivatives and of sodium chloride as impurities. It is preferable, but not essential, that such material is purified before use in the process of the present invention. 4-Formyl-1,3-benzenedisulfonic acid disodium salt (II; Rxe2x95x90SO3xe2x88x92Na+) is typically associated with varying amounts of water. The proportion of such water generally is not critical to the process of the present invention but generally may be taken into account when determining the overall composition of the compound (I)-forming reaction mixture.
The first step is the condensation of the N-tert-butylhydroxylammonium acetate (III) with the aldehyde (II). This reaction is typically conducted in a batch mode with agitation. It could, if desired, be carried out continuously in a flow reaction system.
In this process it is preferred that in general about 1.25 to 2.5 equivalents of N-tert-butylhydroxylammonium acetate (III) is used for each equivalent of the aldehyde (II). It is particularly preferred that about 1.6 to 2.0 equivalents of N-tert-butylhydroxylammonium acetate (III) is used.
The condensation of the present invention is carried out in solution, using a suitable inert solvent in which the starting materials are sufficiently soluble. It is preferred that a suitable polar organic solvent such as an alcohol, or mixture of alcohols, is used as solvent. It is particularly preferred that the solvent is methanol. It is further preferred that the reaction mixture contains a suitable percentage of water, generally less than 10% by volume, such as from about 2% to 10% by volume. It is particularly preferred that the solvent contains about 5% by volume of water. It has been found that the presence of a suitable amount of water provides significant advantages, particularly with regards to inhibiting the conversion of the aldehyde (II) into the undesirable acetal side product (IV) 
by reaction with the solvent R1OH.
The presence of a suitable amount of water in the solvent also increases the solubility of the 4-formyl-1,3-benzenedisulfonic acid disodium salt (II; Rxe2x95x90SO3xe2x88x92Na+) starting material and thereby significantly improves the kinetics of the process and enables the use of a more concentrated system.
The proportion of reaction solvent is typically maintained at about 2 to 8 mL of solvent per gram of nitrone product or greater, with proportions of from 2 to 6 and especially 3 to 4 mL/g being preferred.
The condensation is conducted at a temperature from about ambient temperature to about 150xc2x0 C., good results being achieved at temperatures of from about ambient to about 125xc2x0 C., with temperatures of from about 40xc2x0 C. to about 100xc2x0 C. being preferred.
The condensation reaction is relatively facile and is typically essentially complete in from about 15 minutes to about 5 hours with reaction time of from 30 to 90 minutes being typical. In practice, the degree of reaction is monitored analytically and the reaction is continued until a suitable degree of reaction is achieved.
The isolation of the product of formula (I) formed in the above condensation may be achieved by using standard techniques that are well known in the art. It is particularly advantageous that the product be isolated using a suitable crystallisation technique. Thus in a typical isolation, on completion of the reaction of the aldehyde (II) with N-tert-butylhydroxylammonium acetate (III), the reaction mixture is cooled to ambient temperature and then filtered in order to remove any insoluble material. The filtrate is then adjusted to a temperature that may be from 0xc2x0 C. up to the reflux temperature of the solvent, but is preferably from 35 to 50xc2x0 C., and crystallisation is induced by the addition of a suitable crystallisation agent such as isopropanol or ethyl acetate. The optimal precipitation temperature may vary depending on the scale of the reaction, on whether the suspension is stirred or allowed to stand, and on the desired particle size of the solid product.
The crystallisation agent is typically an organic liquid that is miscible with the reaction solvent but one in which the nitrone product is less soluble. The agent is also generally a volatile material, such as a material having 5 or less carbon atoms. The solid product is isolated by filtration and dried. The use of isopropanol, as a crystallisation agent, is particularly preferred.
Alternatively, crystallisation may be induced by the addition of a suitable agent such as isopropanol or ethyl acetate without the filtrate having first been heated. Again, the use of isopropanol is particularly preferred.
The water content of xcex1-(2,4-disulfophenyl)-N-tert-butylnitrone disodium salt obtained using the process of the present invention is dependent on the nature of the methodology used for the isolation of the product and the final drying process that is used. Thus, extensive drying at elevated temperatures and under reduced pressure will yield essentially anhydrous material. Such material is however significantly hygroscopic, forming eventually a trihydrate. Drying of the trihydrate regenerates the anhydrous form. The trihydrate form is obtained directly by crystallisation of xcex1-(2,4-disulfophenyl)-N-tert-butylnitrone disodium salt from hot water, or by passing humidified air over the solid.
Addition of up to about 5% volume of water to the crystallisation agent can push the product toward the hydrated form and decrease the amount of occluded organic liquids in the crystalline product and can lower impurity levels such as the level of unreacted aldehyde.
The invention is illustrated, but in no way limited, by the following examples.